1. Field of the Invention
The present invention relates to a diffractive optical element and an optical device using the same, and more particularly to a diffractive optical element using a liquid crystal.
2. Description of the Related Art
Active diffractive optical elements using a liquid crystal have been known. As an example of such conventional diffractive optical elements, a configuration of an element disclosed in Japanese Patent Laid-Open No. 2-40615 is shown in FIG. 23.
In FIG. 23, the diffractive optical element includes a transparent substrate 101 having a transparent electrode 102 on one side thereof, a transparent material with a periodic thickness variation 103, another transparent substrate 101 having another transparent electrode 102 thereon, and a liquid crystal 104 sandwiched therebetween. In this case, a liquid crystal layer is about 6 μm thick.
The amplitude of the thickness variation for the material 103 is 50 nm. This uneven structure causes slight differences in thickness of the liquid crystal layer, thereby causing periodic differences in electric field strength when a voltage is applied to the two transparent electrodes 102. The liquid crystal used herein is of a cholesteric/nematic phase change type. It changes from a cholesteric phase to a nematic phase when the applied bias is increased.
A case where the applied voltage decreases to cause transition from the nematic phase to the cholesteric phase will be considered. In the nematic phase, light entering the element transmits the element undiffracted. When the liquid crystal changes to the cholesteric phase, liquid crystal molecules are oriented parallel to the substrate selectively at recesses of the substrate with weak electric fields. This causes periodic refractive index distribution to form a phase diffraction grating.
Japanese Patent Laid-Open No. 2-40615 also discloses another configuration where a transparent electrode is formed on a transparent substrate having an uneven structure previously formed thereon, and a configuration where a transparent electrode having an uneven structure is formed on a surface of a flat transparent substrate. In either configuration, the transparent electrodes are provided across both transparent substrates sandwiching the liquid crystal, and the periodic uneven structure is provided on one transparent substrate, thereby causing slight differences in strength in the electric field distribution inside the liquid crystal layer.
An active diffractive optical element using a liquid crystal with a different configuration is disclosed in Japanese Patent Laid-Open No. 5-72509. A configuration thereof is shown in FIG. 24. In FIG. 24, two transparent substrates 111 provided with transparent electrodes 112 are used to sandwich a liquid crystal 114 similarly as the above described element, but a feature in FIG. 24 is that rectangular transparent solid materials 113 are periodically placed to separate the liquid crystal 114.
For a well-controlled molecular alignment of the liquid crystal, it is preferable to provide alignment treatment on the transparent electrode, though not shown. Specifically, this treatment includes rubbing a polymeric material such as polyimide coated on the surface of the transparent electrode, or oblique deposition of a material such as SiO.
The operation of the element in FIG. 24 is described below. A refractive index of the liquid crystal to a light entering the liquid crystal can be switched between a state of matching a refractive index of the material 113 and a state of not matching it, depending on whether a voltage is applied or not to the two transparent electrodes 112. When the refractive index of the liquid crystal matches the refractive index of the material 113, the light transmits the element undiffracted. When the refractive indices periodically differ, the element operates as a phase diffraction grating. Thus, an optical device such as a switchable filter can be obtained.
Further, a diffractive optical element similar to the element in FIG. 24 is presented in a paper by Sakata, et al., “Switchable zero-order diffraction filters using fine-pitch phase gratings filled with liquid crystals,” (Jpn. J. Appl. Phys. Vol. 39, 2000, pp. 1516–1521).
A configuration described in the paper is shown in FIG. 25. Two transparent substrates 121 with transparent electrodes 122 are used to sandwich a liquid crystal 124. The feature of this configuration is that each of the periodically-placed transparent solid materials 123 has a trapezoidal cross-section.
As in the case shown in FIG. 24, refractive indices of the transparent solid material and the liquid crystal are selected such that a light entering the element in FIG. 25 transmits undiffracted when the voltage is applied, and is diffracted when the voltage is not applied. The cross-section of the transparent solid material is formed into a trapezoidal shape instead of a rectangle to increase an intensity ratio (extinction ratio) of lights that can be switched ON/OFF in a wide visible spectrum range.
In the conventional diffractive optical elements using liquid crystal, a step for forming a film of a transparent electrode material over a transparent substrate, and a patterning step for forming electrode and terminal areas have to be performed on both of the two transparent substrates. In addition, a step for forming the periodic uneven structure or the periodic array of transparent solid materials is required. To form the section of the transparent solid material into the trapezoidal shape or the like, for example, a step for heating and softenning a resist material is added. To orient the liquid crystal molecules to a desired direction, it is preferable to provide alignment treatment such as rubbing or oblique deposition of SiO for example on the transparent electrode. Therefore, it is difficult to reduce manufacturing costs of the conventional active diffractive optical elements using liquid crystal.
Further, to increase a diffraction angle, a finer pitch is required for the periodic structure of the uneven structure or the periodic array of the transparent solid materials. Since the liquid crystal molecules are anchored to the surface of the uneven structure or the transparent solid material, it becomes difficult to change orientations of the liquid crystal molecules. Changing the orientations of the liquid crystal molecules firmly anchored to the surface takes much time, so that a finer pitch of the periodic structure causes reduction in speed for altering the liquid crystal orientation. As a result, for the conventional active diffractive optical elements using liquid crystal, it is difficult to increase the diffraction angle with no response speed penalty.
Further, related arts do not teach arraying many diffractive optical elements to obtain a multi-channel optical device. Specifically, related arts do not teach an approach to reduction in a manufacturing cost per unit channel.
A diffractive optical element is applied for optical devices such as a variable optical attenuator, a polarization separator, an optical switch, a filter, or the like. These optical devices often use optical fibers for guiding light to the diffractive optical element and guiding an output light from the diffractive optical element to an outside. When many diffractive optical elements are arrayed to obtain a multi-channel optical device, the number of optical fibers increases. Therefore, mounting of the optical fibers to the optical device becomes difficult. The related arts do not teach an effective mounting method of the optical fibers.
A power of light entering a diffractive optical element or an intensity of a diffracted light as an output may vary due to some causes that are difficult to control beforehand. For example, a temperature variation can change characteristics of a liquid crystal. A variation in mechanical load to an optical fiber can also cause such a detrimental effect. If a variation of the power is monitored and the voltage applied to the diffractive optical element is adjusted accordingly, there may be provided a diffractive optical element and an optical device using the same that do not depend on these variation factors. However, the related arts do not teach a concept of detecting the intensity of the diffracted light.
If a diffraction angle can be switched as well as the intensity of the diffracted light, the degree of design freedom widens in many applications. However, in the related arts, the pitch of the periodic structures is fixed. Therefore, in the elements shown in FIGS. 23 to 25, the diffraction angle can not be switched.